Polyamide resins are a well known class of resins as is the general method for their preparation. Polyamide resins are typically manufactured by reacting a di- or polyfunctional amine with a di- or polyfunctional acid. Most of the diacids and diamines which are commonly employed to prepare polyamide resins give polyamide resins which are essentially linear. The properties of the polyamide resins will vary considerably depending upon the particular starting reactants used to form the polyamide resin. Polyamide resins which are prepared from relatively short chain diacids and diamines having, for example, 5-10 carbon atoms will tend to be relatively crystalline and have excellent fiber forming properties. These types of polyamide resins are typically referred to as nylons.
Polyamide resins are also prepared from relatively long chain polyfunctional acids and diamines. A particularly important class of polyamide resins of this type are referred to as polymerized fatty acid polyamide resins. The polyfunctional acids used in the preparation of the polymerized fatty acid polyamide resins are derived from higher unsaturated fatty acids by polymerization. In the polymerization process, the fatty acids having double bond functionalities combine to produce a mixture of higher polymeric acids. The polymerized fatty acid polyamide resins are typically prepared by reacting one or more suitable diamines, most commonly relatively short chain diamines, with the polymerized fatty acid. Often another diacid is also reacted to increase the softening point or other properties. The polymerized fatty acid polyamide resins which are obtained tend to be more amorphous than the nylon type of polyamides resins and are generally more flexible. The differences in the physical properties of the polymerized fatty acid polyamide resins as compared to the nylon type of polyamide resins are considered to be related to the long chain length of the polymerized fatty acid component.
The polymerized fatty acid polyamide resins are widely used in a variety of industrial applications. Polymerized fatty acid polyamides are especially useful as hot melt adhesives and for forming water and solvent resistant coatings on substrates such as paper. An important related use of polymerized fatty acid polyamide resins is as binders in printing inks and the like where excellent film toughness, flexibility and adhesion are all important properties.
One of the problems encountered with the polyamide resins and particularly the polymeric fatty acid polyamides is concerned with the methods used to apply the polyamide resins to substrates. One method which has been suggested is to heat the polyamide resins above their melting point and then to apply the molten resin to the substrate. This has certain inherent problems in that polyamide resins typically have a high melting point, which is often higher than the thermal stability of the substrates on which it is desired to apply the molten polyamide resin. The hot melt method accordingly can only be used in certain limited applications and even these applications require relatively expensive application equipment. The use of molten polyamide resins is not practical in many common applications as, for example, in printing. Another example of where it is impractical to use molten polyamide resins is when the polyamide resin is intended to be applied as a latent hot melt layer to be activated at a later time. An example of this type of application is where the polyamide resin is desired to be applied to a textile interliner so that after the garment is completely assembled the hot melted adhesive can be activated to hold the assembled parts of the garment in position.
It was recognized in the art that certain of the problems associated with the polyamide resins could be solved if the polyamides could be applied at ambient temperatures from a solution or a dispersion. Suggestions were made to form organic solvent solutions of the polyamide resins. It was found, however, that for many applications this was unsatisfactory. Polyamide resins as a class have excellent resistance to solvents and even with respect to those solvents in which the polyamide resins are soluble the solubility typically is relatively low. Furthermore, the solvents which were used to make the polyamide resin solutions often adversely reacted with the substrates on which the polyamide resin solutions were applied. A further problem associated with solvent solutions was that the solvents which were used were relatively expensive, were often difficult or impossible to remove from the applied coatings and the solvent presented fire, toxicity and environmental pollution problems.
To overcome, or at least somewhat reduce the problems associated with solvent solutions of polyamide resins it was suggested to prepare emulsions or dispersions of the polyamide resins. The emulsions were prepared by initially dissolving the polyamide resin in an organic solvent and then by using selected emulsification agents form an emulsion of the solvent solution and water. The resulting solvent/water polyamide resin emulsions still had the problems associated with the presence of solvents as noted above for the solvent solutions and were relatively unstable. In addition, films formed from the emulsions tended to have an undesirable tackiness.
In British patent 1,491,136 there was disclosed a method for forming aqueous dispersions of various plastic powders, including polyamide resin powders. In the disclosed method, the polymer resin was first mechanically reduced to a powder form and then blended with water and a thickening agent. The method was less than satisfactory. The mechanical reduction of the resins to the required particle size was both expensive and difficult to control and often caused thermal degradation of the polymers. Furthermore, the resulting thickening dispersions had limited utility in many applications because of the relatively high viscosity and the presence of the thickening agent.
It was also suggested in the prior art to chemically modify the polyamide resins so as to include solublizing groups to make the modified resins more readily dispersible in water. This included, for example, incorporating groups such as alkoxymethyl as disclosed by Carirns in U.S. Pat. No. 2,430,860 and a related patent by Watsen et al in U.S. Pat. No. 2,714,075. The incorporation of the additional groups into the polyamide resin increased the cost of the polymer and also typically reduced the desirable properties of the polyamide resins especially in relation to water and solvent resistance.
Another method which was suggested for increasing the water dispersibility consists of forming the polyamide resin so that there is a considerable excess of either free carboxyl or free amine groups. At least a portion of the free acid or free amine groups were then neutralized to form salt groups on the polyamide resin. The resulting salt groups on the polyamide resin then acted as an internal surfactant which facilitates the dispersion of the modified polyamide in water. In Wittcoff et al U.S. Pat. No. 2,811,459 there is disclosed a method to form polymerized fatty acid polyamide dispersions wherein the polyamide is formed from a substantial excess of a diamine. The resulting polyamide resins are then dispersed in an aqueous solution of an acid so that the acid forms salt groups which act as the internal surfactant to allow formation of an aqueous dispersion. In Wittcoff et al U.S. Pat. No. 2,768,090 a process similar to that described above is disclosed wherein the excess amine groups of a polyamide resin are reacted with an acid to form an intrinsic ammonium salt group so as to be able to form a cationic dispersion which then is converted to an anionic dispersion by charge inversion. A similar salt forming process utilizing free amino groups was disclosed in Wittcoff U.S. Pat. No. 2,824,848. In Wittcoff U.S. Pat. No. 2,926,117 there is disclosed a method wherein the polyamide resin formed with a deliberate excess of acid groups is then dispersed in an aqueous medium containing an alkaline substance to thereby cause formation of salt groups to act as internal surfactants.
The above described methods for preparing aqueous dispersions of polymerized fatty acid polyamides where the polyamide is synthesized with substantial amounts of free amine or carboxylic acid groups, which are thereafter converted to their respective salt groups, is a relatively effective method for initially forming aqueous dispersions, but the dispersions have limited stability and are not satisfactory for use in many applications. These processes require the presence of substantial amounts of free acid or free amino groups for the salt formation. This has a definite adverse effect on the properties of the dispersed polyamide resin. The polymerized fatty acid polyamide resins are especially useful in products such as hot melt adhesives, water resistant coatings, printing inks and the like because of their physical properties including high strength, excellent flexibility, water and solvent resistancy, and the ability to form smooth non-tacky coatings. The optimum desired properties are achieved by conducting the amidations so as to cause as complete as a reaction as possible. This requires that approximately stoichiometric amounts of the starting diacid and diamine be employed and that the reaction be conducted so as to allow for complete reaction and formation of the amide groups so that the final product will have a low amine and low acid number. The presence of substantial excesses of either reactant or the incomplete reaction, as required for the prior art salt forming polyamide material, inherently reduces the chain length and the resulting strength and flexibility of the polyamide resin. It is also highly desirable in many applications where the polymeric fatty acid polyamide resins are employed, such as in hot melt applications or in coating applications, that the polyamide which is used be stable with regard to temperature and storage and have a narrow melting point range. These particular properties are best obtained with a completely reacted polyamide resin. The polymerized fatty acid polyamide resins with substantial amounts of free acid and amino groups such as those used in the prior art processes are unstable for these applications in that the modified polyamide resins continue to react during application which causes increases in the molecular weight, increases in the coating viscosities, and changes in the melting point. A still further problem encountered with the method wherein the salt forms of the polyamide resins are used is that the salts tend to decompose during application and the resulting material when applied becomes undesirably tacky. This is particularly undesirable in many applications such as in printing inks and protective coatings. Furthermore, because of the relative instability of the materials caused by substantial excess amounts of unreacted polymerized fatty acids in the polymer the fatty acids can be liberated from the polymer and cause exceptional tackiness and undesirable degradation of the desired properties of the polyamide resin. Because of the problems associated with the polymerized fatty acid polyamide resins having the salt groups formed as part of the polymer, the aqueous dispersions of these particular types of resins have had no substantial commercial success.
Accordingly, it would be highly advantageous if a method could be provided for forming stable aqueous dispersions of non-solvated, un-neutralized, low acid, low amine number polymerized fatty acid polyamide resins.